Fiber Morphology
In the pulp and paper industry all processing apparatuses and methods are influenced by the pulp characteristics, which derive particularly from fiber morphology, population, length, coarseness, coarseness to length ratio, wall thickness and diameter, and fiber types to list the most important ones.
Fiber morphology is the key element that influences the pulp characteristics. Up to now the paper industry has not given fiber morphology full consideration as regards pulp processing and papermaking.
Among the most important aspects that affect the morphology of the fibers are the species of the trees or non-wood plants that are processed into pulp. The huge difference between hardwood or softwood pulps is best illustrated by the consistent difference in their fiber populations. Thus fines-free softwood market pulps range from 2.3 to 5.2 million fibers/g, while fines-free hardwood pulps range from 9.8 to 27.6 million fibers/g. In terms of fiber length, the average length-weighted fiber length of softwood chemical pulps range from 1.8 to 2.7 mm, while the shorter hardwoods range from 0.58 to 1.1 mm. These data show that softwood fibers are, on average, approximately 2.5 times longer than hardwood fibers. There is a sharp difference in the paper structure that these two groups of pulp produce, and in fact most papers are made from a mixture of hardwood and softwood pulps.
Another major aspect of fibrous pulp is coarseness, a measure of how much fiber wall material there is in a fiber per unit-length of that fiber. Since the fiber wall densities of market chemical pulps do not differ greatly, coarseness can be visualized as the solid cross-sectional area of the fiber wall. Once more a significant difference between softwood and hardwood fibers is evident. Coarseness values for the softwoods range from 13.7 to 27.5 mg/100 m, while hardwoods range from 6.9 to 12.6 mg/100 m.
The relationship of fiber diameter and wall thickness affects the tendency of the fiber to collapse. Fibers with the same coarseness, but different diameters, will have different collapsing tendencies. A good example of this difference can be seen by comparing a 100% aspen NBHK, and a SBMHK, “Southern Bleached Mixed Hardwood Kraft” pulp. They have nearly the same coarseness values (10.1 versus 10.6 mg/100 m), but the aspen pulp fibers have a larger diameter and a thinner wall than the southern mixed hardwood fibers. As a result, the aspen pulp collapses readily and therefore requires only a third as much refining to form a sheet with the same density as the southern hardwood pulp.
Pulp fines are generally considered to be detrimental to pulp properties because they don't contribute significantly to sheet strength and they displace longer fibers, they lower freeness and impede the drainage on the paper machine, they aren't easily retained on the paper machine wire, they tend to produce paper surfaces prone to picking and dusting, and they have a high specific surface area that attracts a disproportionate amount of additives.
There is another category of fines, called secondary fines that are fragments of fibers and fiber walls. Secondary fines result from fiber damage in wood chip production and transport, pulping, mixing and refining. Secondary fines, within some reasonable limits, are generally considered beneficial for sheet strength, opacity and surface properties.
As regards non-wood pulps, cotton linters pulps are nearly pure cellulose fibers produced from second-cut cotton linters. The distribution of fiber lengths, average fiber length (1.45 mm), coarseness (19.1 mg/100 m) and fiber population (5.5 million/g) position this pulp's gross morphology as being similar to short softwood. From a strength potential perspective, the nearly pure cellulose severely limits hydration and bonding potential. As a consequence refining has little influence on strength development and, while these pulps are quite durable and stable, they are not used to make strong papers without being combined with other fibers and/or bonding additives.
Three other non-wood pulps belong to the category of pulps with very uneven fiber length distribution and quite different fiber types. Bamboo, kenaf and bagasse pulps have very similar fiber length (˜1.1 mm), but their coarseness values, which are 9.3, 12.5 and 17.9 mg/100 m respectively, cover a fairly broad range. All three pulps have at least two very distinct fiber types, namely a long, relatively thick-walled fiber much like softwood latewood fiber, and a large diameter thin-walled short fiber.
Wood Cell Structure
Another difference between softwoods and hardwoods is that softwoods are dominated by one fiber type, the tracheid, while hardwoods have fiber tracheids and vessel elements. The vessel elements specialize in vertical transportation of water in the tree trunk. Each vessel consists of vessel elements that are cylindrical cells with open ends. They are connected longitudinally to form pipe structures. All hardwoods have vessels in the range of 10% to 50% of their volume, depending on species. Vessel elements of some hardwood species are extremely large in diameter compared to their fibers. In addition to having more fiber types, the volume that the fiber types occupy, and their size and shape, is more varied in hardwoods.
In softwoods the tracheids are 90+% of the wood volume and 94+% of the pulp mass. In contrast, the hardwood fiber tracheid volume ranges from 40 to 70%, while the vessel elements occupy 10 to 40% of the wood.
There is a third group of small cells, namely the ray tracheids in softwoods and the ray and axial parenchyma in hardwoods. Ray cells constitute less than 10% of the volume of softwoods, but the parenchyma cells in hardwoods can 30 be up to 40% of the wood volume and more than 20% of the mass of a market pulp. These very small cells are commonly referred to as primary fines.
To illustrate the ultrastructure of typical softwood cells reference is made to FIG. 1 The main cell (fiber) type is the axially aligned tracheid (TR) 302. Other cell types in softwoods are the fusiform wood ray (FWR) and wood ray (WR) cells 304, and the longitudinal and epithelial parenchyma cells, which are the cells surrounding the horizontal (HRO) 306 and vertical (VRO) 308 resin ducts.
A simplified structure of a cylindrical woody cell, i.e. a wood fiber, comprises the middle lamella (ML) 312, the primary wall (P) 314, the outer (81) 316, middle (82) 318, and inner (83) 320 layers of the secondary wall, and the warty layer (W) 322.
The warty layer 322 is a thin, typically 0.1 micron thick, amorphous membrane located in the inner surface of the cell wall. In the inner layer 320 the direction 10 of the micro-fibrils is nearly perpendicular to that in the middle layer 318. The void space inside the inner layer 320 is called lumen 321 and its diameter is typically in the 20 micron range.
The middle layer 318, typically 3 micron thick, forms the main portion of the cell wall. The micro-fibrils are oriented in bands (lamellae) nearly parallel to the cell axis.
In the outer layer 316, typically 0.2 micron thick, the micro-fibril groups are in helixes alternately crossed. The fibrils have great tensile strength, which renders the outer layer 316 capable of extreme resistance to swelling of the interior layers of the fiber wall.
In the outer portion of the primary wall 314 the micro-fibrils form an irregular network. In the interior they are oriented nearly perpendicular to the cell axis. In the presence of reagents (e.g. white liquor), which induce strong swelling, the primary wall 314, typically 0.1 micron thick, is peeled off and the belts around the fibers expand (balloon).
The middle lamella 312 is located between the cells and serves the function of binding the cells together. It consists typically of 90% lignin, 10% of hemicelluloses, and little, if any, of cellulose.
In softwoods, liquid is transferred from rays 304 to tracheids 302 and between tracheids 302 through tiny voids called bordered pits (BP) 310 in the cell walls. Typically the bordered pits 310 are only in one single row aligned with the longitudinal axis of the cell.
To give some perspective of the weight each cell type and their layers represent in a virgin softwood cell structure, typically the percentages are in descending order as follows: middle layer 318 67%, middle lamella 312 15%, wood rays 304 and resin ducts 306 and 308 (including resins) 8%, outer layer 316 5%, primary wall 314 3%, and inner layer 320 2%.
Pulping
The major types of fibrous pulps produced today are chemical pulps made by digesting, thermomechanical pulps (TMP) made by heat and mechanical defibrillation and chemi-mechanical and chemi-thermomechanical pulps (CMP and CTMP) made by mechanical refining with use of heat and/or chemicals. Chemical pulping has a low yield and the pulps are rather expensive. Mechanical pulps are rather weak and yield lower-quality although cheaper paper products compared with chemical pulps. Ultra-high-yield chemi-mechanical (CMP) pulps and chemi-thermo-mechanical (CTMP) pulps represent alternatives, which combine high yield, low cost, and fewer pollution problems while achieving good mechanical properties. The only major drawback of CMP/CTMP pulps is their relatively high defibrillation energy.
The production of CMP or CTMP generally comprises the steps of chip sulfonation with a sodium sulfite mixture (NaHS03/Na2S03/NaOH) and high-consistency refining at atmospheric or higher pressure.
Explosion pulping is an ultra-high-yield pulping process based on short time vapor-phase cooking at temperatures in the range of 180 to 21 OOC, followed by explosive decompression.
It is generally accepted that the chemical treatment is mainly responsible for permanent fiber softening and increases in long fiber content, fiber specific surface, and conformability. Treatment of chips with steam at high temperature, followed by explosive decompression, also contributes to the softening of the fibers. Moreover, the explosive decompression of treated chips leads to a reduction in energy consumption in subsequent refining stages.
Explosion pulping was invented by Mason (1928) in the early 1930s. In this process, the chips are fed from a chip bin through a screw loading valve in a masonite gun. The chips are then steam heated at a very high temperature, about 285OC, and at a pressure of 3.5 MPa for about 2 min. The pressure is increased rapidly to about 7 MPa for about 5 s, and the chips are then discharged through restricted orifices (slotted port) and explode at atmospheric pressure into a pulp. Two stages of low-consistency atmospheric refining result in a dark pulp of about 75% yield suitable for the manufacture of high-density fiberboard. This process is very effective as a means of fiber separation with low energy consumption. However, the masonite pulp is very coarse in texture and dark in color. The fibers are mostly un-collapsed, rigid, and degraded and display a highly lignin-rich surface structure, which is unsuitable for papermaking.
Scanning electron micrographs and microscopic studies of fibers produced by explosion pulping have demonstrated a clear separation in the middle lamella. It is known that the surface of explosion pulp contains numerous large pores with evidence of considerable ultra-structural rearrangements. Fragments of cell wall form more or less spherical particles, primarily from lignin. During the action of organic solvents or other delignifying agents, the lignin particles swell, then their structure is destroyed, and finally they move from the liquid phase and disappear from the fiber structure.
Steam explosion pulping (SEP) was suggested as an alternative to CMP/CTMP processes in the early 1990s because of the reduced refining energy required for the SEP pulp. A much higher temperature is used (180-210° C.) as compared with that used in conventional CMP/CTMP processes (120-150° C.) and a shorter time. Moreover, the cook is terminated by a sudden pressure release.
Paper recycling is the process of recovering waste paper, pulping the paper and turning it into new paper products. There are three categories of paper that can be used as feed stocks for making recycled paper, namely mill broke, pre-consumer waste, and post-consumer waste. Mill broke is paper trimmings and other paper scrap from the manufacture of paper, and is recycled internally in a paper mill. Pre-consumer waste is material which left the paper mill but was discarded before it was ready for consumer use. Post-consumer waste is material discarded after consumer use, such as corrugated containers, magazines, newspapers, office paper, telephone directories etc.
Pulp Processing
The most usual processes fibrous wood or non-wood pulps are subjected to washing, bleaching, chemical treatments of the pulp, refining, drying, and papermaking. Recycled fibrous wood or non-wood pulps are subjected to deinking, bleaching, refining and papermaking.
Pulp washing aims at the removal of residual digesting or bleaching liquor from the pulp. Washing apparatuses include diffusion washers, rotary drum washers, horizontal belt filters, wash presses and dilution/extraction washing stages. The method used in diffusion washers is to displace in a tank the spent liquor between the fibers with cleaner wash liquor, typically at 5-10% pulp consistency range. The method used in drum and belt-washers is to first dewater the pulp slurry as much as possible, typically to 10-15% pulp consistency range, and then use wash liquor for displacement of the remaining spent liquor. In the method with wash presses the spent liquor is squeezed out from between the fibers, typically to 25-35% pulp consistency range, while some wash liquor is introduced during the pressing.
All these displacement washing methods displace spent liquor only from the space outside and between the pulp fibers, but leave practically all spent liquor inside the fiber lumen. Some leaching of the inside bound liquor happens during the dilution/extraction stages between multiple washing steps in series. However, a substantial amount of spent liquor, typically 1.5-2 times the weight of the fibers, is still carried over inside the hollow fiber lumen to the next processing stage.
Bleaching of pulp fibers is done with chemicals in gaseous or liquid form. One or more bleaching stages are required to achieve desired result. Each bleaching stage is accomplished at a preferred pulp consistency, temperature, pressure, time duration, and chemical concentration.
Each bleaching stage is followed by a washing stage to remove the dissolved organic matter and the bleaching chemicals to minimize the carry-over to the next bleaching stage. In bleaching typically only one washing step is used per stage. Again, the displacement wash leaves a substantial amount of already dissolved organic matter and bleaching chemicals inside the fiber lumen. This carry-over consumes and thereby wastes some of the newly added chemicals in the next bleaching stage.
In general, recycled pulp can be bleached with the same chemicals used to bleach virgin pulp, but hydrogen peroxide and sodium hydrosulfite are the most common bleaching agents.
The industrial process of removing printing ink from paper fibers of recycled paper to make deinked pulp is called deinking. Many newsprint, toilet paper and facial tissue grades commonly contain 100% deinked pulp and in many other grades, such as lightweight coated for offset and printing and writing papers for office and home use, deinked pulp makes up a substantial proportion of the furnish.
In the fast-growing digital printing market, a noteworthy development is the introduction of commercial inkjet web presses for on-demand newspapers and various publications and business forms. However, inkjet inks are generally not de-inkable and are, therefore, incompatible with paper recovery and recycling. Ten percent is a reasonable estimate for the percentage of inkjet-printed paper that a mill can tolerate.
Very small and hydrophilic ink pigments are generally easily dislodged from the fiber surface during pulping. The mechanical action imparted to the pulp in a repulper, however, causes the small ink pigments to enter into the lumen via pit apertures where they deposit irreversibly on the surface of fiber lumen.
Some end products in chemical pulping require additional chemical treatment after the conventional bleaching stages. For instance, mercerized pulps are produced by post-treatment of steeping a bleached pulp in caustic to swell the fibers, to remove hemicelluloses and to render the pulp inert as far as strength development from refining is concerned. Market pulps are used to produce cellulose derivatives including sodium carboxyl methyl cellulose (CMC), hydroxyl ethyl cellulose (HEC), methyl ethyl cellulose (MEC) and cellulose diacetate.
Dissolving pulps having uniform intrinsic viscosity, also measured as degree of polymerization, undergo a process that reduces it to syrup that can be further processed into cellophane film and fibers for rayon, acetate and other man-made fibers.
Refining or beating of chemical pulps is the mechanical treatment and modification of fibers so that they can be formed into paper or board of the desired properties. The main target of refining is to improve the bonding ability of fibers so that they form strong and smooth paper sheet with good printing properties.
Refining affects fibers by cutting and shortening of fibers, fines production, removal of parts from fiber walls, external fibrillation, delamination, internal fibrillation, swelling, curling, creating kinks etc., and dissolving or leaching out colloidal material into the external liquor. As a result of the above effects, fibers after refining are collapsed (flattened) and made more flexible, and their bonding surface area is increased.
Production of market pulp as well as various types of paper and board products requires drying of the final product. Most market pulp is air dried (90% fiber, 10% water) and compressed into bales or in some cases the pulp mat is split and rewound into rolls. Two basic systems are employed in the production of dry market pulp: the conventional system, and the flash-drying system.
The conventional system of producing dry market pulp parallels conventional papermaking: a thick pulp mat is formed with a Fourdrinier wet end, most remaining free water between the fibers is removed mechanically in the press section, and evaporative drying is employed with either a steam-heated cylinder dryer or an air-float dryer section.
Flash or spray drying refers to the process whereby the fibrous material is introduced as a spray or an analogous form into a stream of hot gases. The high-temperature heat content of the gas stream causes flashing of moisture to vapor.
However, the entrapped water in the lumen immediately turns to steam (one cc of water becoming 1700 cc of steam), creating an internal explosion. The water in the fiber's wall and interior lumen evaporates, causing the fiber to shrink and contract. Each fiber reacts to this explosion in varying ways, depending on the structural weaknesses along the length of the fiber. These inherent weaknesses create knuckles in the fibers' shape. Along with the knuckles the fibers develop a characteristic curly shape and become convoluted. These misshapen fibers have made flash-dried pulps recognized as being ideally suited to the needs of those mills producing filter papers and latex-saturated paper and board.
In drying a sheet of paper two basic physical processes are involved, heat transfer and mass transfer. Heat is transferred from some source such as steam to the wet sheet in order to provide the energy required to drive the moisture from the sheet. The moisture evaporates and is then transferred from the sheet to the surrounding atmosphere by the mass transfer process. Multicylinder drying is the most common way in the paper and paperboard industry.